Automatic power transfer switch

ABSTRACT

A static switch is operated upon sensing a loss of a primary power supply for providing a rapid temporary connection of a secondary power supply to a load. Slower acting power contactors are also energized and their operation provides a permanent connection of the secondary power supply to the load and also provides means for disconnecting the path of secondary power through the static switch.

United States Patent Ireland et al.

[54] AUTOMATIC POWER TRANSFER SWITCH [72] Inventors: Ralph 11. Ireland, Warminster; Edward C.

Lesoravage, Doylestown, both of Pa.

[73] Assignee: The United States of America as represented by the Secretary 01' the Navy [22] Filed: May 19,1970

[21] Appl.No.: 38,813

[52] US. Cl ..307/64, 307/136, 317/11 E [51] Int. Cl. ..H02j 9/06 [58] Field oiSearch ..307/136, 64; 317/1 1 E, 31

[56] References Cited UNITED STATES PATENTS 3,401,303 9/1968 Walker ..307/ 136 X 3,201,592 8/1965 Reinert et a1. ..307/64 3,154,695 10/1964 MacGregor et al.. ....307/262 X 3,237,030 2/1966 Coburn ..307/136 X CONTACTOR AC. POWER SOURCE POWER SOURCE 1 Feb. 29, 1972 3,293,446 12/1966 Baude.... ..307/64 X 3,321,668 5/1967 Baker..... ....307/l36 X 3,372,303 3/1968 Kn0tt..... ....307/136 X 3,408,538 10/1968 Gurwitz ....307/136 X 3,421,070 1/1969 Ettinger ....307/136 X 3,449,591 6/1969 Hoover ....307/136 3,515,896 6/1970 Swing et al ..307/64 Primary Examiner-Robert K. Schaeffer Assistant Examiner-William J. Smith Attorney-R. S. Sciascia and Henry Hansen [57] ABSTRACT A static switch is operated upon sensing a loss of a primary power supply for providing a rapid temporary connection of a secondary power supply to a load. Slower acting power contactors are also energized and their operation provides a permanent connection of the secondary power supply to the load and also provides means for disconnecting the path of secondary power through the static switch.

8 Claims, 1 Drawing Figure POWER SOURCE A.C. POWER SOURCE PATENTEDFEB 29 m2 MQZDOw mmBOm 0 womnow mm30m Ud INVENTORS RALPH H. IRELAND EDWARD c. LESORAVAGE mQmDOm mwiOa Ud ATTORNEY wQmDOm mm30m zohoqhzoo AUTOMATIC POWER TRANSFER SWITCH STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates generally to power supply systems with more than one source of power available and more particularly to automatic bus transfer systems.

In an automatic transfer system it is often necessary to achieve the automatic transfer of power to a second AC power source within a shorter duration of time than is practical with automatic electromechanical switches when the first power source fails or becomes disconnected. If the transfer of power sources is through the use of electromechanical contactors, it may take up to 50 milliseconds for the complete mechanical operation. Furthermore, transient conditions may exist due to contact chatter or bounce. oftentimes due to requirements of the load this transfer is not rapid enough to keep the load operating in a smooth continuous manner. It therefore becomes necessary to provide a system which is capable of making a smoother transfer in a much shorter period of time.

SUMMARY OF THE INVENTION Accordingly, it is the purpose of the present invention to provide a system which upon sensing a loss of primary power is capable of providing an alternate power source to the load in approximately 3 milliseconds. This is accomplished by paralleling the normal contactors that would provide the secondary load with a static transfer switch. This transfer switch upon sensing the loss of power provides a temporary path for the secondary power supply through the static switch until the contactots that are normally used are able to operate and the subsequent operation of these contactors connects the alternate power source to the load and disconnects the static transfer switch.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE shows a block schematic diagram of an embodiment of the present invention.

' DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the FIGURE of the drawing, there is shown a main three-phase power source connected to a load 11 through a contactor 12. Connected individually to each phase of the three-phase power supply are the anodes of diodes 15, 16 and 17. The cathodes of diodes 15, 16 and 17 are tied together and connected to the parallel combination of capacitor 18 and resistor 19 which are in turn tied together at their opposite contacts and connected to ground. The base 22 of PNP-transistor 23 is also connected to the cathodes of diodes l5, l6 and 17. A series connection of diode 26 and resistor 27 is connected between the emitter 28 and base 22 of transistor 23. The collector29 of transistor 23 is connected to one side of the primary winding of transformer 31. The other side of the primary winding of transformer 31 is connected to a resistor 32 that, in turn, is connected to ground. Connected across the primary winding of transformer 31 is a diode 33. The secondary winding of transformer 31 is connected across the gate 37 and cathode 38 of silicon-controlled rectifier 39. The anode 40 of SCR 39 is connected to the emitter 28 of transistor 23 and the diode 26. A'resistor 45 is connected between cathode 38 and ground. Cathode 38 of SCR 39 is connected to the series circuit of resistor 41a and capacitor 424 whose other contact is connected to ground. The emitter 43a of unijunction transistor 44a is connected between resistor 41a and capacitor 42a. A resistor 47a is connected between cathode 38 and base 8, of UJT 44a. Base B of UJT is connected through the primary winding of transformer 51a to ground. Resistors 41a and 47a, capacitor 424, UJT 44a and the primary winding of transformer 51a comprise relaxation oscillator 55a.

The secondary winding of transformer 51a is connected across the cathode 52a and gate 53a of SCR 540. SCR 540 and diodes 57a, 58a, 59a and 60a form a bridge circuit 61a with the anode 62a of SCR 54a connected between diodes 59a and 60a and the cathode 52a of SCR 54a connected between diodes 57a and 58a. A conductive lead 630 connects bridge 61a to load 11. A conductive lead 64a connects the other side of bridge 61a to an alternate power source 67. It is therefore seen that the power source 67 is connected to load 11 through bridge 61a. Although only one bridge 61a on one of the phases of the power supply 67 has been described it is to be noted that the other two phases of the power supplies have similar bridges 61b and 610 and associated control devices having similar numeric representation with the addition of the letters b or c. The alternate power source 67 has another connection to load 11 through a latching relay contactor 68 with spring latch 91 that has normally open contacts 82a, 82b and 82c that parallel circuits 61a, 61b and 610.

An interlock relay 70 has a normally open contact 71 connected to the emitter 28 of the transistor 23. Connected to the other side of the contact 71 is a manually operated switch 72 which is connected to a separate DC control power source 73. Connected between the contact 71 and manual switch 72 is the control winding 74 of the interlock relay 70. Connected to the other side of the control winding 74 is a normally closed contact 75 of contactor 68. The other side of contact 75 is grounded. The cathode 38 of SCR 39 is also connected to normally open contact 76 of interlock relay 70. The other side of normally open contact 76 is connected to normally closed contact 77 which in turn is connected to test switch 78 which is connected to receive DC power source 79. Connected between contacts 76 and 77 is the control winding 81 of contactor 68. The other side of the control winding is grounded.

The operation of the device will now be described with reference to the FIGURE. In normal operation the power supply 10 supplies power to load 11 through contactor l2. Diodes 15, 16 and 17 provide a DC signal to capacitor 18. resistor l9, and the base 22 of transistor 23. From the polarity of diodes 15, 16 and 17 it can be seen that the DC signal supplied is of positive polarity and this signal provided to the parallel connection of capacitor l8 and resistor 19 is of sufficient magnitude to prevent transistor 23 from conducting. Switch 72 is closed for automatic operation and DC control power source 73 supplies power to interlock relay control winding 74 through switch 72 and contact 75. This operates relay 70 and provides DC power through contact 71 to the emitter 28 of transistor 23. However, this voltage soui'ce is not of sufiicient magnitude to overcome the voltage on the base 22 and as a result transistor 23 is nonconductive. SCR 39 is likewise non conductive as there is no signal applied to gate 37 and, therefore, no power is supplied through contact 76 to contactor control winding 81 and as a result contactor 68 remains open. During this normal operation the test switch 78 is kept in the open position.

Upon failure of power supply 10 the diodes l5, l6 and 17 fail to provide a power source to transistor 23, capacitor 18 and resistor 19. Capacitor 18 then rapidly discharges through resistor 19 causing a drop in the voltage applied to the base 22. The sole source of power to transistor 23 then becomes power source 73 and it can be seen that between emitter 28 and base 22 a voltage drop is obtained across diode 26 and resistor 27 that biases transistor 23 into a conduction state. The initial conduction of transistor 23 pulses transformer 31 into turning on SCR 39.

The operation of only one of the bridge circuits and its control circuitry will now be explained. It is to be understood that the other bridge circuits and control circuitry operate in a similar manner.

The conduction of DC power source 73 through SCR 39 causes a high-frequency oscillation of relaxation oscillator 55a. Therefore, UJT 44a provides a rapidly oscillating signal to transformer 51a. The frequency of this signal is approximately 20,000 Hertz as compared to the normal power supply of 400 Hertz. The secondary of transformer 5 la then supplies a signal to SCR 54a which causes SCR 540 to fire. It is therefore seen that on the positive cycle from alternate power supply 67 a conduction path is available through diode 59a, SCR 540, and diode 57a. n negative half cycles a power supply is available through diode 58a, SCR 54a, and diode 60a. Hence, the system provides a power supply from alternate power supply 67 through bridge 61 to load 11 while contactor 68 is open.

Simultaneous to switching bridge 61 into conducting current from alternate power source 67 to load 1 1, control power source 73 also supplies power to contactor control winding 81 through contact 71, SCR 39 and contact 76. However, contactor 68 is a slower acting device than static bridge 61 and as a result its contacts do not close for approximately 50 milliseconds as compared to a 3-millisecond operating time of bridge 61 Upon operation of contactor 68 alternate power supply 67 is provided with a path directly through contacts 82a, 82b and 820 to load 11. In addition, the operation of contactor 68 disconnects the circuit path through bridge circuits 61a, 61b and 610. The opening of the circuit path through bridge 61a will now be explained it being understood bridge circuits 61b and 61c operate in a similar manner.

Upon the closing of contactor 68 the normally closed contact 75 of contactor 68 opens. This in turn deenergizes control winding 74 of relay 70. The opening of contact 76 removes the power source 73 from the system. As a result SCR 54a ceases to fire and there is no longer a circuit path through bridge 61a.

The test circuit operates in the following manner. Upon closing test switch 78 the DC power supply 79 is applied to contactor control winding 81. Upon operation of contactor 68 the main contacts 82a, 82b and 820 on the powerline close and contact 75 opens disconnecting DC control power 73 from the system as described previously.

It is therefore seen that a static switch can be provided to rapidly connect a secondary source of power upon the failure of the main power source. It is further shown that this static switch can be automatically disconnected upon the operation of the electromechanical contactors that make a permanent connection of the alternate power supply to the load. In this way an alternate power source may be connected to a load in approximately 3 milliseconds compared to the normal 50-millisecond delay time of a normal electromechanical contactor.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is: I

1. An automatic transfer switching system comprising:

a power source;

sensing means connected to said power source for providing a first signal upon sensing a voltage loss from said power source;

first switching means connected to receive said first signal for establishing a first current path upon receipt of said first signal having a plurality of relaxation oscillators connected to said sensing means for providing a plurality of second signals upon receipt of said first signal, and silicon-controlled rectifier bridges to receive said second signals for providing said first current path upon receipt of said second signals; and

second switching means connected to receive said first signal for subsequently establishing a second current path and interrupting said first current path upon receipt of said first signal.

2. An automatic transfer switching system according to claim 1 wherein said second switching means further comprises:

an electromechanical switch connected in parallel with said silicon-controlled rectifier bridges for providing said second current path.

3. An automatic transfer switching system according to claim 2 wherein said second switching means further comprises:

first relay means connected to said sensing means for closing a first plurality of normally open contacts on said electromechanical switch to provide a second current path and opening a normally closed contact on said electromechanical switch upon receipt of said first signal; and

second relay means connected in series with said normally closed contact to interrupt said first current path upon opening of said normally closed contact.

4. In combination with a first power source for providing power to a load through a first electromechanical contactor and a second power source for automatically providing power to said load through a second electromechanical contactor upon failure of the first power source the improvement of which comprises:

a plurality of silicon-controlled rectifier bridges connected in parallel with said second contactor;

means for firing said bridges to establish a first current path from said second power source to said load upon failure of said first power source; and

means for subsequently establishing a second current path from said second power source to said load through said second electromechanical contactor and for interrupting said first current path.

5. A power system comprising:

a first power supply;

a second power supply;

sensing means connected to said first power supply for providing a first signal upon sensing a failure of said first power supply having a plurality of diodes for rectifying said first power supply, a parallel combination of a capacitor and resistor connected to said plurality of diodes for receiving said rectifier signal, a transistor having its base connected to said diodes receiving said rectified signal, a DC power supply connected for providing a DC signal to the emitter of said transistor, a series combination of a diode and resistor connected between said emitter and said base of said transistor, a transformer having its primary winding connected to the collector of said transformer, and a silicon-controlled rectifier having its gate and cathode connected across the secondary winding of said transformer, the anode of said SCR connected to said DC power supply.

6. A power system according to claim 5 wherein said first switching means further comprises:

a plurality of relaxation oscillators connected to said sensing means for providing a plurality of second signals upon receipt of said first signal; and

silicon-controlled rectifier bridges to receive said second signals for providing said first current path upon receipt of said second signals.

7. An automatic transfer switching system according to claim 6 wherein said second switching means further comprises:

an electromechanical switch connected in parallel with said silicon-controlled rectifier bridges for providing said second current path.

8. An automatic transfer system according to claim 7 wherein said second switching means further comprises:

first relay means connected to said sensing means for closing a first plurality of normally open contacts on said electromechanical switch to provide a second current path and opening a normally closed contact on said electromechanical switch upon receipt of said first signal; and

second relay means connected in series with said normally closed contact 'to interrupt said first current path upon opening of said normally closed contact.

at k 

1. An automatic transfer switching system comprising: a power source; sensing means connected to said power source for providing a first signal upon sensing a voltage loss from said power source; first switching means connected to receive said first signal for establishing a first current path upon receipt of said first signal having a plurality of relaxation oscillators connected to said sensing means for providing a plurality of second signals upon receipt of said first signal, and siliconcontrolled rectifier bridges to receive said second signals for providing said first current path upon receipt of said second signals; and second switching means connected to receive said first signal for subsequently establishing a second current path and interrupting said first current path upon receipt of said first signal.
 2. An automatic transfer switching system according to claim 1 wherein said second switching means further comprises: an electromechanical switch connected in parallel with said silicon-controlled rectifier bridges for providing said second current path.
 3. An automatic transfer switching system according to claim 2 wherein said second switching means further comprises: first relay means connected to said sensing means for closing a first plurality of normally open contacts on said electromechanical switch to provide a second current path and opening a normally closed contact on said electromechanical switch upon receipt of said first signal; and second relay means connected in series with said normally closed contact to interrupt said first current path upon opening of said normally closed contact.
 4. In combination with a first power source for providing power to a load through a first electromechanical contactor and a second power source for automatically providing power to said load through a second electromechanical contactor upon failure of the first power source the improvement of which comprises: a plurality of silicon-controlled rectifier bridges connected in parallel with said second contactor; means for firing said bridges to establish a first current path from said second power source to said load upon failure of said first power source; and means for subsequently establishing a second current path from said second power source to said load through said second electromechanical contactor and for interrupting said first current path.
 5. A power system comprising: a first power supply; a second power supply; sensing means connected to said first power supply for providing a first signal upon sensing a failure of said first power supply having a plurality of diodes for rectifying said first power supply, a parallel combination of a capacitor and resistor connected to said plurality of diodes for receiving said rectifier signal, a transistor having its base connected to said diodes receiving said rectified signal, a DC power supply connected for providing a DC signal to the emitter of said transistor, a series combination of a diode and resistor connected between said emitter and said base of said transistor, a transformEr having its primary winding connected to the collector of said transformer, and a silicon-controlled rectifier having its gate and cathode connected across the secondary winding of said transformer, the anode of said SCR connected to said DC power supply.
 6. A power system according to claim 5 wherein said first switching means further comprises: a plurality of relaxation oscillators connected to said sensing means for providing a plurality of second signals upon receipt of said first signal; and silicon-controlled rectifier bridges to receive said second signals for providing said first current path upon receipt of said second signals.
 7. An automatic transfer switching system according to claim 6 wherein said second switching means further comprises: an electromechanical switch connected in parallel with said silicon-controlled rectifier bridges for providing said second current path.
 8. An automatic transfer system according to claim 7 wherein said second switching means further comprises: first relay means connected to said sensing means for closing a first plurality of normally open contacts on said electromechanical switch to provide a second current path and opening a normally closed contact on said electromechanical switch upon receipt of said first signal; and second relay means connected in series with said normally closed contact to interrupt said first current path upon opening of said normally closed contact. 